1. Field of the Invention
The present invention relates to an optical constant calculation method and a substrate processing system, and more particularly, to a method for calculating optical constants of a plurality of films layered on a substrate and a substrate processing system in which the method is implemented.
2. Description of the Related Art
With advancement of downsizing of semiconductor devices, a circuit pattern is required to be more finely formed on a surface of a wafer used for the fabrication of semiconductor devices. To form a fine circuit pattern on a wafer having a plurality of films layered thereon, a wafer surface structure (for example, a wafer surface structure comprised of etched films) must be identified during the semiconductor device fabrication.
To identify the surface structure of an etched wafer, there has been known a method for observing and photographing a cleaved cross-section of the wafer using an SEM (scanning electron microscope). However, this method has a drawback that the wafer must be cut (destroyed) along the cross section to be observed.
To nondestructively identify the surface structure of an etched wafer, methods have been developed in which the wafer surface structure is identified by scatterometry such as reflectometry, ellipsometry, or the like generally used for resist pattern evaluation or the like (see, for example, Japanese Laid-open Patent Publication No. 2002-260994).
In particular, the reflectometry (as scatterometry) nondestructively identifies the wafer surface structure using n-values (refractive indexes) and k-values (damping coefficients), which are optical constants representing the wafer surface structure. More specifically, optical constants (n-values and k-values) of films layered on a surface of each wafer are calculated in advance. These layered films include, for example, a nitride film, an organic insulation film (low-k film), an oxide film, an antireflection film (BRAC film), and a resist film. Next, using the calculated optical constants of the films, models are prepared and stored each of which optically represents, e.g., a groove shape (more generally, a surface structure) of one of the wafers which are different in groove shape. Then, the reflectivity of a surface of a wafer whose surface structure is to be identified is measured. By selecting a groove shape model corresponding to the measured reflectivity, the surface structure (groove shape) of the wafer is identified (see, for example, Japanese Laid-open Patent Publication No. 2005-33187).
With the scatterometry, the wafer surface structure cannot be identified with accuracy when the calculated optical constants of the films are not accurate. To obviate this, it is extremely important to accurately calculate the optical constants of the films.
In the conventional optical constant calculation method, the reflectivity or the like of each of films to be layered on a wafer surface is measured immediately after each film is formed, and the optical constant of each film is then calculated. For the calculation of the optical constant of the formed film, the optical constant of an underlayer film of the formed film is used. The optical constant of the underlayer film is generally calculated immediately after the underlayer film is formed.
However, during the layered film formation, the density of the underlayer film whose optical constant has already been calculated can sometimes be changed when heat treatment is performed for formation of an upper film. In general, the optical constant changes with a change in density. This indicates that the optical constant of the underlayer film used for the calculation of the optical constant of the upper film is made different from the real optical constant of the underlayer film, resulting in inaccuracy of the optical constant of the upper film, which is calculated using the already calculated optical constant of the underlayer film.
The underlayer film is altered when it is etched, so that the optical constant thereof can further be changed. In a case where the wafer surface structure including the etched underlayer film is identified by reflectometry, the optical constant of the underlayer film used for the calculation of the optical constant of the upper film differs from the real optical constant of the underlayer film. Therefore, the wafer surface structure identified using the already calculated optical constant of the underlayer film becomes inaccurate.